There has been used a technique for comparing (i) input image data obtained by reading a document image with a scanner and (ii) a preliminarily stored image, so as to determine a similarity between the input image data and the preliminarily stored image, and controlling a process (such as copy, transmission, and edition) to the input image data in accordance with the result of the determination.
Examples of a method for determining a similarity include: a method in which a keyword is extracted from an image with OCR (Optical Character Reader) so as to carry out matching with the keyword; a method in which a target image is limited to an image with ruled lines and matching is carried out based on features of the ruled lines (see Patent Document 1 (Japanese Unexamined Patent Publication No. Tokukaihei 8-255236 (published on Oct. 1, 1996))); and a method in which a similarity is determined based on distribution of color components of input image data and a reference image (see Patent Document 2 (Japanese Unexamined Patent Publication No. Tokukaihei 5-110815 (published on Apr. 30, 1993))). Furthermore, Patent Document 3 (International Publication No. 2006/092957, pamphlet (published on Sep. 8, 2006) discloses a technique in which plural feature points are extracted from a digital image, a set of local feature points are determined out of the extracted feature points, a partial set of feature points is selected out of the determined set of local feature points, invariants relative to geometric transformation each as a value characterizing the selected partial set is calculated in accordance with plural combinations of feature points in the partial set, features are calculated by combining the calculated invariants, and a document or an image with the calculated features in a database is voted for, thereby searching a document or an image corresponding to the digital image.
Furthermore, an example of a technique for controlling a process for input image data in accordance with the result of similarity determination is as follows: in order to prevent forgery of paper currency or stock certificate by a color image forming apparatus, it is determined whether input image data is identical with image data such as paper currency or stock certificate (reference image) or not based on a pattern extracted from the input image data, and when the input image data is identical with the image data of the reference image, a specific pattern is given to an output image so as to specify the image forming apparatus that carries out copying, a copied image is daubed, or copying is prevented.
Furthermore, Patent Document 4 (Japanese Unexamined Patent Publication No. Tokukaihei 11-284847 (published on Oct. 15, 1999)) discloses a technique in which it is determined whether input image data is identical with a reference image (specific image) or not, and when it is determined that the input image data is identical with the reference image, a notification that the input image data is identical with the reference image is sent to an apparatus administrator, thereby preventing a normal copy output of the reference image. Furthermore, Patent Document 4 discloses that, when the apparatus administrator finds that the input image data is not identical with the reference image after receiving the notification, the apparatus administrator changes a determination level for determining the reference image.
Furthermore, Patent Document 5 (Japanese Unexamined Patent Publication No. Tokukai 2002-232707 (published on Aug. 16, 2002)) discloses a technique in which it is determined whether input image data is identical with image data of a document (reference image) such as paper currency or not, and when it is determined that the input image data is identical with the image data of the reference image, an image forming function is restricted so as not to perform a normal image formation, and (i) information that the reference image is tried to be printed and (ii) circumstance information indicative of a detailed circumstance where the reference image is tried to be printed (such as date, the number of sheets tried to be printed, scaling, an image tried to be printed, a log for a recognition process, and a partial image recognized as being illegal) are sent to other apparatus. Furthermore, Patent Document 5 discloses that, when the log for the recognition process and the partial image recognized as being illegal are sent as the circumstance information, these information is encrypted before being sent.
In an image processing apparatus for determining whether input image data is identical with image data of a reference image or not (determining a similarity between an input image and a reference image), limitation of accuracy in reading an image etc. may result in erroneous determination depending on conditions under which the image is read.
For that reason, in order to ensure higher security, it is possible to set a small threshold value for a similarity at which input image data is determined as being identical with a reference image. However, in that case, there is a higher possibility that input image data that is not identical with image data of a reference image is erroneously determined as being identical with the image data of the reference image. This may result in restriction of a process on input image data that is not to be restricted in fact, or may result in frequent notification to the administrator due to erroneous determination, increasing a workload on the administrator. Thus, a convenience for the user or the administrator drops.
On the other hand, when the threshold value is set to be large, there is a higher possibility that input image data that is identical with image data of a reference image is erroneously determined as not being identical with the image data of the reference image, which results in problems such as lower security.
Therefore, it is necessary to set the threshold value depending on which of security and convenience is considered as being more important.
Which of security and convenience is considered as being more important may vary according to a circumstance condition of input image data (such as user, date, day of the week, time (time zone), a process mode, the amount of data to be processed, and whether image data with a specific hue is included or not).
For example, there is a case where convenience is considered as being important for a user satisfying a predetermined condition (e.g. a user belonging to a specific department in a corporation, a preliminarily stored user) whereas security is considered as being important for a user not satisfying the predetermined condition, so as to prevent an unspecified user from performing a process on an image that may be identical with a reference image, thereby preventing illegal use of the reference image or leakage of the reference image.
Further, there is a case where convenience is considered as being important at a predetermined date, a predetermined day of the week, or a predetermined time (e.g. normal working day, normal working time, preset date, preset day of the week, and preset time in a corporation) whereas security is considered as being important at date, day of the week, or time other than the predetermined date, the predetermined day of the week, or the predetermined time (e.g. holiday and nighttime), so as to limit date, day of the week, or time at which an image that may be identical with a reference image can be processed, thereby preventing use or leakage of the reference image.
Further, there is a case where security is considered as being important for a predetermined process mode (e.g. transmission to network, copy, printing) whereas convenience is considered as being important for a process other than the predetermined process mode, so as to limit a process mode that can be carried out for an image that may be identical with a reference image, thereby preventing illegal use or leakage of the reference image.
Further, there is a case where convenience is considered as being important for data with not more than a predetermined amount (e.g. predetermined number of documents, predetermined number of customer information) whereas security is considered as being important for data with more than the predetermined amount, so as to limit the amount of data that can be processed with respect to an image that may be identical with a reference image, thereby preventing a large amount of leakage of the reference image.
Further, a confidential document is often stamped with characters such as “confidential” written in a predetermined color such as red or blue. Therefore, there is a case where security is considered as being important when input image data includes a predetermined color image. Whereas, there is a case where convenience is considered as being important when input image data does not include the predetermined color image.
However, in the techniques of Patent Documents 1 to 5, determination for a similarity between an input image and a reference image is always performed on the same reference (the same threshold value), and as a result it is impossible to perform a similarity determination process according to a circumstance condition for input image data.